The invention relates to magneto-caloric materials, and more particularly to regenerators for magneto-caloric regeneration.
Conventional refrigeration technology has often utilized the adiabatic expansion or the Joule-Thomson effect of a gas. However, in case of such gas compression technology, a refrigerant material is used. Examples of such refrigerant materials may include a hydro-fluorocarbon (HFC), a hydro-chlorofluorocarbon (HCFC), or a chlorofluorocarbon (CFC) gas. If not disposed off properly, the refrigerant material may pose environmental challenges. Additionally, the gas compression technology is a mature technology and extracting additional energy savings out of this technology has proved difficult.
An alternative refrigeration technique involves a magnetic refrigeration method that uses entropy change accompanied by a magnetic or magneto-structural phase transition of a magneto-caloric material, referred to as a magnetic phase transformation. In the magnetic refrigeration technique, cooling is effected by using a change in temperature resulting from the entropy change of the magneto-caloric material. In particular, the magneto-caloric material used in the magnetic refrigeration method alternates between a low magnetic entropy state with a high degree of magnetic orientation created by applying a magnetic field to the magnetic material near a transition temperature (typically near Curie temperature) of the magnetic material, and a high magnetic entropy state with a low degree of magnetic orientation (randomly oriented state) that is created by removing the magnetic field from the magnetic material. Such a transition between high and low magnetic entropy states manifests as a transition between low and high lattice entropy states, in turn resulting in warming up or cooling down of the magneto-caloric material when exposed to magnetization and demagnetization. This is known as the “magneto-caloric effect.” It is desirable to leverage the magneto-caloric effect present within certain magneto-caloric materials to develop a magnetic refrigerator.
Conventional magneto-caloric material based systems require heat exchangers (or regenerators) for heat transfer between the magneto-caloric material and the heat exchange fluid. Magneto-caloric materials include multiple alloys that are typically brittle and have a tendency to become powders due to inherent stress in the material. Moreover, magneto-caloric materials have low thermal conductivity and hence are less efficient when subjected to transient operating cycles due to cyclic magnetization and demagnetization. Conventional heat exchanger designs use porous bed structures that have a high pressure drop. Additionally, the porous bed structures are prone to erosion.
Further, during operation, regenerator components need to be in physical contact with organic or aqueous based coolants. Based on the nature of the coolant (acidic or basic), magneto-caloric materials of the regenerator components react with the coolant. For example, a magneto-caloric material when directly exposed to the aqueous heat exchange fluids reacts to form oxide or hydroxide layers on a surface of the magneto-caloric material, which in turn may lower the efficiency and reliability of the heat exchanger in magneto-caloric refrigeration systems. The oxides and/or hydroxides on the surface of the magneto-caloric materials may cause degradation in the heat transfer coefficient. With time, this oxide/hydroxide spalls from the magneto-caloric materials thereby causing enhanced resistance to fluid flow. Moreover, due to magnetic cycling, cracking followed by disintegration of the magneto-caloric materials is highly probable.